Molecular complexes between antigenic peptides and proteins of the Major Histocompatibility Complex play a key role in immunological discrimination between self and non-self This research entails studies of the kinetics of reactions between class II MHC proteins and peptides, with particular reference to the formation of isomers of these complexes. The class II proteins to be studied included water soluble constructs of murine E(k) and human DR molecules. The peptides are generally tagged with fluorescent groups for high sensitivity detection, or fluorine atoms for NMR spectroscopy. It is theoretically possible that defects in the discrimination between self and non-self lead to autoimmune disease, and that such defects may sometimes be related to the adventitious formation of isomers of class II-MHC peptide complexes.